Below is a list of countries which use the definite article. In English, we use
"the"if a country has a political title in its name, or if it refers a group of islands. There are also countries, such as the Netherlands, which people commonly attach the definite article to even though it does not follow the two rules above.
For bodies of water, we frequently add the definite article to rivers, oceans, and seas.
For single mountains, we don’t typically add the definite article. However, for groups of mountains, we do.
You’ll find many examples of these rules below, as well as a list of other geographical features which use the definite article. You can do an exercise HERE